[NOTE: This review pertains specifically to the free public domain ebook edition offered in the Kindle Store, though its remarks are relevant to any other edition to which it may (unintentionally and erroneously) be attached.]
This free, public domain edition is the "classic" old translation by Louis Mercier most of us have read and loved. It is a grand adventure and is very enjoyable in a quaint, old-fashioned way. There is nothing terrible with this popular translation, but (admittedly) chunks of the novel WERE left out, not so much that the overall story itself was greatly altered, but still leaving the finished work not as Verne wrote it and intended it to be. (What was omitted were essentially scientific details Mercier thought impeded the story, some possibly awkward political references, and sections presumably thought to be redundant.)
But you can decide for yourself the extent to which these omissions may have negatively impacted the story, because, fortunately, there is ANOTHER free, public domain version available which I would encourage you to ALSO download and read; that one is a modern translation by F. P. Walter and it is unabridged. It may be found in the Kindle store by typing: "Verne Vingt Mille English." The title is French but don't worry, the book is entirely in English with a very informative introduction by Walter. This great new translation is wordier than the old one, but it comes as close (in English) to what Verne actually wrote (in French) -- and it IS complete; in fact, it has about 100 more pages! I would strongly urge you to compare them (especially if you are a true Jules Verne fan). But quite frankly, EITHER version tells a great story.
FYI: Be aware of two other excellent, new, complete, modern translations of "20,000 Leagues" available in the Kindle Store, one by Anthony Bonner (Random) and another by Mendor Brunetti (Penguin). Both are very good, though I personally lean toward Brunetti's purely for stylistic reasons. You may wish to sample and compare both of them. Unlike Walter's translation, Bonner's and Brunetti's are not in the public domain and they are not free, but both are very reasonably priced (with Brunetti's being the cheaper of the two).